There are multiple ways to approaching this question...
There a great number of non-D&D rpgs out there. Not only are there multiple games out there in print, but there are piles of out-of-print games at used books stores. How many of them are "successful" is open to huge debate; as a game-consumer, a "successful game" is one that I run and enjoy, but that definition wouldn't work for most game companies.
D&D is the top dog, but not to quite the same level as in the earlier days. Go into any Borders or Barnes & Noble and you can see White Wolf games as well as WotC; in several of them you can find Battlestar Galatica and/or Serenity as well. Due to this, there is a slightly more open vision of rpgs for the general public, a notion that they are more than simply D&D.
D&D is still the top do, as I said, and will remain that way for quite some time, perhaps as long as tabletop rpgs exist. It has a huge advantage over the other games -- it's name is synonymous with rpgs. In many ways it is the Starbucks or McDonalds of rpgs. It is ubiquitous and, in many ways, the only fully accessible tabletop rpg out there -- you can find it all over the place, while other games you have to go searching. As such it tends to cycle in on itself -- since it is more available, it is the game that most gamers start with ... and thus becomes the "standard" by which other games are measured ... and as it is the one most people learn first (and learning a game requires a noticeable amount of time and desire) it remains the most popular. This is NOT to say it is not a good game; I don't mean that at all, but it does mean that its ubiquity becomes a bit more difficult for other systems to gain traction, even games that specific gamers might actually prefer (or not ... some people try other games and still quite happily go back to D&D).
Savage Worlds is doing okay; NWoD is doing fine; GURPS has fallen down in this incarnation, though exactly why I am uncertain; SotC did very well when it first came out and continues all right, but the sales appear to have tapered off; Battlestar Galactica and Serenity sell, much as does Star Wars, even to non-gamer "completists", thus even if the numbers are not to the D&D level, they are certainly acceptable.
